The Wilson Journal 
of Ornithology 
Published by the Wilson Ornithological Society 
VOL. 123, NO. 1 March 2011 PAGES 1-198 
The Wilson Journal of Ornithology 123(1): 1-14, 2011 
SPECIES LIMITS IN ANTBIRDS (THAMNOPHILIDAE): THE 
SCALE-BACKED ANTBIRD (WILLISORNIS POECILINOTUS) COMPLEX 
MORTON L. ISLER 13 AND BRET M. WHITNEY 1 2 
ABSTRACT.—The geographic range of the Scale-backed Antbird (Willisornis poecilinotus) encompasses Amazonia. 
Seven currently defined subspecies are distinguished from one another by diagnostic plumage characters except for one 
pair. Six pairs of subspecies are apparently parapatric and lack a known barrier to intergradation in at least a portion of their 
contact zone; yet confirmed hybrids are known only for one pair in one location. An analysis of >350 recordings, however, 
found vocal differences among them insufficient to recommend elevating subspecies to the species level with one 
exception. Populations in southeastern Amazonia should be considered a distinct species, Willisornis vidua (Hellmayr), 
Xingu Scale-backed Antbird, on the basis of their distinct loudsongs, raspy call series, and contact calls. Within the 
widespread Willisornis poecilinotus , Common Scale-backed Antbird, the remaining instances of parapatry without 
extensive intergradation provide a focus for future fieldwork to define interrelationships in contact zones and mechanisms 
of species recognition that may be sustaining them on independent evolutionary paths. Received 75 May 2010. Accepted 9 
September 2010. 
The Scale-backed Antbird ( Willisornis poecili¬ 
notus) (Cabanis 1847), a widespread Amazonian 
complex, occupies a unique place in thamnophilid 
antbird evolution. Scale-backed Antbirds, as 
described in detail by Willis (1982), primarily 
forage over or near army ant swarms, but their 
morphology directed early taxonomists (Ridgway 
1911, Cory and Hellmayr 1924) to place them in 
the genus Hylophylax with species that were not 
obligate ant-followers. Willis (1982) and other 
observers (e.g., Zimmer and Isler 2003) noted that 
Scale-backed Antbirds did not look or behave like 
1 Department of Vertebrate Zoology, MRC-116, National 
Museum of Natural History, Smithsonian Institution, P. O. 
Box 37012, Washington, D.C. 20013, USA. 
2 Museum of Natural Science, 119 Foster Hall, Louisiana 
State University, Baton Rouge, LA 70803, USA. 
^Corresponding author; e-mail: antbird@cox.net 
other Hylophylax species, but it remained for a 
molecular study (Brumfield et al. 2007) to 
demonstrate that Scale-backed Antbirds evolved 
in the clade of army ant-following birds distant 
from Hylophylax species in the phylogenetic tree. 
The genus Dichropogon had been erected earlier 
(Chubb 1918) for the complex, but Agne and 
Pacheco (2007) found the name Dichropogon was 
preoccupied by a genus of asilid flies and 
proposed the new generic name of Willisornis in 
honor of Edwin O'Neill Willis, who had contrib¬ 
uted so much to the understanding of the complex 
as well as other thamnophilid species. 
Seven subspecies have been recognized (Peters 
1951, Zimmer and Isler 2003). Almost all are 
readily distinguished by plumage features, pri¬ 
marily in females. Substantial differences in 
female plumage led Hellmayr (1929) to include 
the complex in his seminal study of geographic 
1 
